1. Field of the Invention
The present invention relates to a portable HMI lighting fixture. More particularly, but not by way of limitation, the present invention relates to portable HMI lighting fixtures for use in the motion picture and television industries having an integral, high frequency ballast.
2. Background of the Invention
HMI lighting equipment enjoys widespread acceptance in the motion picture and television industries. Perhaps the two primary factors for the acceptance of HMI lighting are:
a) the color temperature of the light produced by an HMI bulb is very close to daylight; and
b) HMI lights are very efficient compared to tungsten lights. Unfortunately, like other types of arc lamps, HMI bulbs require a ballast to limit the electrical current flowing through the bulbs and an ignitor is required to initiate the arc.
Portable lighting plays an important role in the motion picture and television industries. Unlike theater lighting, where a lighting installation is typically long term and in place for a number of performances, motion picture lighting is usually setup for a relatively short period of time and, even during the short time, a particular lighting arrangement is in place, the individual lights often undergo numerous adjustments in position, height, angle, intensity, and the like.
While HMI lights are presently used for temporary lighting, they suffer from a number of limitations which adversely impact their functionality. For example, as previously mentioned, HMI lights require a ballast. Due to the size and weight of a typical ballast, the ballast is traditionally housed in a separate enclosure. A cable then connects the fixture to the ballast and a second cable is used to connect the ballast to incoming AC power. In addition, ignition of the bulb is handled in a number of different ways. Ignitors are typically placed in or near the fixture but occasionally may be found in the ballast or along the wiring between the fixture and the ballast. Depending on the ballast, an ignitor pulse may be generated either automatically or by manual actuation of a switch. Furthermore, ignition of a hot bulb requires an ignition sequence different from that of a cold bulb which, with many ballasts, results in an unacceptable amount of time to re-strike the arc.
These limitations might best be understood by comparing HMI lighting to tungsten lighting. A typical tungsten light is housed in cylindrical housing. A reflector behind the bulb, or integral with the bulb, ensures that the bulk of the light produced will exit the front of the housing. A Fresnel lens is often secured over the front of the housing to provide a degree of focusing, thereby allowing the light to be aimed at a particular spot. The tungsten bulb connects directly to AC power and a simple on/off switch may be provided to activate or deactivate the bulb. Dimming may be accomplished by simply plugging the light into a conventional, off-the-shelf light dimmer. In contrast to HMI lamps, tungsten bulbs employ a filament which may be extinguished and re-activated at will simply by reapplying power to the bulb. Thus, tungsten lights are easy to set-up and simple use. Unfortunately, the color temperature of a tungsten light is not close to daylight; the color temperature varies widely with dimming or variations in the line voltage; and the bulbs are not particularly efficient.
A typical HMI light is likewise housed in a cylindrical housing. A reflector is usually provided behind the bulb, or integral with the bulb, to ensure that the bulk of the light is directed towards the front of the housing. A Fresnel lens may be secured over the front of the housing to provide a degree of focusing, likewise allowing the light to be aimed at a particular spot. However, in contrast, the HMI bulb cannot be directly connected to AC power. Instead, the ballast must be provided between the bulb and AC power and an ignitor is required to xe2x80x9cstrikexe2x80x9d the bulb to initially start the arc. Due to its size, weight, and sensitivity to heat, the ballast in present HMI lighting equipment for the motion picture and television industries is physically separated from the fixture, thus limiting the portability of HMI lighting equipment.
HMI ballasts are available in a variety of configurations. As with ballasts for other types of arc lamps, HMI ballasts can be grouped broadly into two categories, namely: a) magnetic ballasts; and b) electronic ballasts. Magnetic ballasts limit the current passing through the bulb by providing an inductor between the HMI bulb and the power line, relying on the impedance of the inductor at the line frequency (e.g., 60 Hz in the U.S. or 50 Hz in Europe) to limit current through the bulb. The size and weight of a magnetic ballast is dictated by the inductor which is typically rather large and heavy. Thus, magnetic ballasts are particularly inappropriate for use with portable lights.
Electronic ballasts, generally speaking, rectify the voltage from the AC line into a DC voltage. The DC voltage is then xe2x80x9cchoppedxe2x80x9d at a relatively high frequency, typically in the range of 20,000 Hz to 200,000 Hz. An inductor is then placed between the high frequency output and the bulb to limit the current passing through the bulb. At this higher frequency, the inductor will be of a radically smaller value and, therefore, of much smaller physical size than the inductor incorporated in a magnetic ballast. Typically, this reduction in the size and weight of the inductor more than offsets the size and weight of the additional electronic components required for rectification and chopping, resulting in a substantially smaller and lighter ballast than its magnetic counterpart.
It should be noted that, in setting up lighting equipment for use in filming, the light fixtures are often mounted on stands or suspended from trusses. In either case, the light fixtures are typically manually lifted into place. Thus, adding the weight of a magnetic ballast and increasing the size of the light fixture to accommodate such a ballast would severely degrade the functionality of the fixture. It should also be noted that while an electronic ballast may not increase the size and weight of the fixture to unacceptable levels, such ballasts are typically sensitive to the extreme heat produced within the light fixture by the bulb.
Thus, it can be seen that there is a need for a HMI lighting equipment which provides the benefits of HMI lighting but may be setup and operated with the ease of a tungsten light.
The present invention provides an HMI light fixture with integral ballast which satisfies the needs and alleviates the problems mentioned above. The inventive apparatus comprises: a housing having a front portion and a rear portion; a lamp socket housed in the front portion; an HMI lamp removably engaged in the socket; an electronic ballast secured in the rear portion; and a thermal barrier separating the front portion and the rear portion.
In one embodiment, an ignitor is provided in the housing such that the bulb is automatically ignited as power is applied to the ballast. An on/off switch is provided on the housing, or in a cord for connecting power to the fixture, allowing activation or deactivation of the HMI light by simply changing the state of the switch. Furthermore, it is preferable that the ballast employed in the inventive fixture includes hot re-strike capability to further simplify operation of the HMI light.
The size and weight of an electronic ballast are inversely related to the frequency of operation of the ballast. For HMI ballasts providing adequate power output, presently the state-of-the-art limits the upper end of the frequency range to about 200 kilohertz. At this frequency, the size and weight of a ballast required to drive an HMI bulb becomes relatively small compared to the other components such as the housing, socket, and the bulb itself. Thus, an HMI light fixture may be constructed which includes an integral ballast but which provides the ease of handling of tungsten fixtures.
Locating a ballast in the proximity of the bulb has also been limited by the heat produced by both the bulb and the ballast. The use of fans is impractical in fixtures of the inventive type since audio is often recorded along with the filming of images. Thus, the ballast must be thermally isolated from the bulb to maintain the electronic components at an operable temperature. In the inventive device, a thermal barrier separates front and rear portions of the housing to thermally isolate the ballast from the HMI bulb. The thermal barrier preferably includes a reflector to direct radiant heat away from the rear portion and an insulative barrier further separates the front and rear portions to reduce the heat transmitted to the rear portion of the housing through conduction. Finally, the thermal barrier restricts air flow between the front and rear portions to reduce heating of the rear portion through convection.
Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following description of the preferred embodiments.